What is the difference between 나는 and 저는 and in what situation are words used? I was reading somewhere that 저는 is used when speaking to someone older. By being humble and polite by lowering yourself to the other person, is that the case or is it wrong?

Why are there two pronunciations for the numbers 4, 7 and 9 in Japanese? I have always been curious to know if there was a specific reason why (if there is any). Thank you for answering my question :-) they were very helpful.